Never allow dither for non-legacy (sRGB or F16) targets.

With sRGB, the noise from dithering is (obviously) non-linear, and
highly objectionable for very dark values. With F16, it's completely
unnecessary.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2113793002

Review-Url: https://codereview.chromium.org/2113793002
1 file changed